2.1 Lymphocytic Choriomeningitis Virus Research and Human Disease
Lymphocytic choriomeningitis virus (LCMV), a member of the family arenaviridae, is a prototypic mouse model virus in research on viral infections. Since its isolation in the 1930s (Rivers and McNair Scott, 1935, Science, 81(2105): 439-440) studies using this virus have uncovered many key concepts in viral immunology and pathogenesis (summarized in Zinkernagel, 2002, Curr Top Microbiol Immunol, 263:1-5; Oldstone, 2002, Curr Top Microbiol Immunol, 263:83-117). LCMV has been extensively used to investigate viral molecular biology and immune responses particularly in the context of persistent infection. The natural host of LCMV are mice, however, several reports revealed that LCMV might also be a neglected human pathogen (Barton, 1996, Clin. Infect. Dis, 22(1):197; Wright et al., 1997, Pediatrics 100(1): E9). Moreover, numerous other members of the arenavirus family have been found in rodent populations around the world. In addition to the Old World arenavirus Lassa virus (LASV), which can be found in Africa, several New World arenaviruses like Junin (JUNV), Guanarito or Machupo are prevalent in diverse rodent populations of South America (Johnson et al., 1966, Am J Trop Med Hyg, 15(1): 103-106; Tesh et al., 1993, Am J Trop Med Hyg 49(2):227-235; Mills et al., 1994, Trop Med Hyg 51(5): 554-562). Upon transmission to humans, many of those viruses can cause viral hemorrhagic fever associated with high mortality (Geisbert and Jahrling, 2004, Nat Med 10(12 Suppl): S110-121).
2.2 Genomic Organization of Lymphocytic Choriomeningitis Virus
Arenaviruses are enveloped viruses. Their genome consists of two segments of single-stranded RNA of negative sense (L: 7.2 kb, S: 3.4 kb). Each segment encodes for two viral genes in opposite orientations. The short segment (S segment) encodes the viral glycoprotein (GP) precursor (GP-C; 75 kDa) and the nucleoprotein (NP; 63 kDa) (Salvato et al., 1988, Virology 164(2): 517-522). The long segment (L segment) expresses the RNA-dependent RNA polymerase (RdRp; L protein; approximately 200 kDa) and the matrix protein Z (protein Z), a RING finger protein (11 kDa) (FIG. 1A) (Salvato et al., 1988, Virology 164(2): 517-522). The GP precursor GP-C is post-translationally cleaved into GP-1 and GP-2, which remain non-covalently associated (Buchmeier and Oldstone 1979, Virology 99(1): 111-120). Trimers of GP-1 and GP-2 are assembled as spikes on the surface of virions and are essential for mediating entry into the host cells by interaction with the cellular surface receptors. Binding and entry of the virus into host cells was long claimed to be mediated by interaction of the LCMV GP with the cellular receptor α-Dystroglycan as the only cellular receptor for LCMV (Cao et al., 1998, Science, 282(5396):2079-2081). Only very recently three additional human molecules (Axl and Tyro3 from the TAM family and dendritic cell-specific intracellular adhesion molecule 3-grabbing nonintegrin) were postulated as additional receptors for LCMV and LASV, a close relative of LCMV, which enable entry of LCMV into cells independently of α-Dystroglycan (Shimojima and Kawaoka 2012, J Vet Med, 74(10):1363-1366; Shimojima et al., 2012, J Virol 86(4):2067-2078). NP binds to the viral RNA, forming the nucleocapsid, which serves as a template for the viral L protein. The nucleocapsid associated with the viral L protein forms the so-called ribonucleoprotein complex, which is active both in replication and transcription and represents the minimum unit of viral infectivity. It has been shown, that NP and the L protein are the minimal trans-acting factors necessary for viral RNA transcription and replication (Lee et al., 2000, J Virol 74(8): 3470-3477). The two genes on each segment are separated by a non-coding intergenic region (IGR) and flanked by 5′ and 3′ untranslated regions (UTR). The IGR forms a stable hairpin structure and has been shown to be involved in structure-dependent termination of viral mRNA transcription (Pinschewer et al., 2005, J Virol 79(7): 4519-4526). The terminal nucleotides of the UTR show a high degree of complementarity, resulting in the formation of secondary structures. These panhandle structures are known to serve as the viral promoter for transcription and replication, and their analysis by site-directed mutagenesis has revealed sequence- and structure-dependence, tolerating not even minor sequence changes (Perez and de la Torre, 2003, Virol 77(2): 1184-1194).
2.3 Reverse Genetic System
Isolated and purified RNAs of negative-strand viruses like LCMV cannot directly serve as mRNA i.e., cannot be translated when introduced into cells. Consequently transfection of cells with viral RNA does not lead to production of infectious viral particles. In order to generate infectious viral particles of negative-stranded RNA viruses from cDNA in cultured permissive cells, the viral RNA segment(s) must be trans-complemented with the minimal factors required for transcription and replication. With the help of a minigenome system which has been published several years ago, viral cis-acting elements and transacting factors involved in transcription, replication and formation of viral particles could finally be analyzed (Lee et al., 2000, J Virol 74(8): 3470-3477; Lee et al., 2002, J Virol 76(12): 6393-6397; Perez and de la Torre 2003, J Virol 77(2): 1184-1194; Pinschewer et al., 2003, J Virol 77(6): 3882-3887; Pinschewer et al., 2005, J Virol 79(7): 4519-4526). Also for other arenaviruses like LASV and Tacaribe virus reverse genetic systems have been established (Lopez et al., 2001, J Virol 75(24): 12241-12251; Hass et al., 2004, J Virol 78(24): 13793-13803). Two publications showed the recovery of infectious LCMV entirely from cDNA using pol-I/-II or T7/pol-II-driven plasmids, respectively (referred to as “viral rescue”) (Flatz et al., 2006, Proc Natl Acad Sci USA 103(12): 4663-4668; Sanchez and de la Torre, 2006, Virology 350(2): 370-380).
2.4 Recombinant LCMV Expressing Genes of Interest
The generation of recombinant negative-stranded RNA viruses expressing foreign genes of interest has been pursued for a long time. Different strategies have been published for other viruses (Garcia-Sastre et al., 1994, J Virol 68(10): 6254-6261; Percy et al., 1994, J Virol 68(7): 4486-4492; Flick and Hobom, 1999, Virology 262(1): 93-103; Machado et al., 2003, Virology 313(1): 235-249). In the past it has been shown that it is possible to introduce additional foreign genes into the genome of bi-segmented LCMV particles (Emonet et al., 2009, PNAS, 106(9):3473-3478). Two foreign genes of interest were inserted into the bi-segmented genome of LCMV, resulting in tri-segmented LCMV particles (r3LCMV) with two S segments and one L segment. In the tri-segmented virus, published by Emonet et al., (2009), both NP and GP were kept in their respective natural position in the S segment and thus were expressed under their natural promoters in the flanking UTR (FIG. 1B). However, the present application reveals that the tri-segmented LCMV particle disclosed by Emonet et al., assembles predominately bi-segmented particles (i.e., the arenavirus only packages one instead of two S segments), resulting in attenuated growth and strong selection pressure to recombine the two S segments. As further shown in the present application, such recombination is reproducibly found and results in phenotypic reversion to wild-type virus and transgene loss.
2.5 Replication-Defective Arenavirus
Recently, it has been shown that an infectious arenavirus particle can be engineered to contain a genome with the ability to amplify and express its genetic material in infected cells but unable to produce further progeny in normal, not genetically engineered cells (i.e., an infectious, replication-deficient arenavirus particle) (International Publication No.: WO 2009/083210 A1 and International Publication No.: WO 2014/140301 A1).